rts: More const correct-ness fixes

In addition to more const-correctness fixes this patch fixes an
infelicity of the previous const-correctness patch (995cf0f356) which
left `UNTAG_CLOSURE` taking a `const StgClosure` pointer parameter
but returning a non-const pointer. Here we restore the original type
signature of `UNTAG_CLOSURE` and add a new function
`UNTAG_CONST_CLOSURE` which takes and returns a const `StgClosure`
pointer and uses that wherever possible.

Test Plan: Validate on Linux, OS X and Windows

Reviewers: Phyx, hsyl20, bgamari, austin, simonmar, trofi

Reviewed By: simonmar, trofi

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2231

1 files changed, 7 insertions, 2 deletions

diff --git a/rts/Hash.h b/rts/Hash.h
index c2dfc26d6d..2d0c5588c5 100644
--- a/rts/Hash.h
+++ b/rts/Hash.h
@@ -13,10 +13,15 @@
 
 typedef struct hashtable HashTable; /* abstract */
 
-/* Hash table access where the keys are StgWords */
+/* Hash table access where the keys are StgWords.
+ * Values are passed into the hash table and stored as `const void *` values,
+ * but when the value is looked up or removed, the value is returned without the
+ * `const` so that calling function can mutate what the pointer points to if it
+ * needs to.
+ */
 HashTable * allocHashTable    ( void );
+void        insertHashTable ( HashTable *table, StgWord key, const void *data );
 void *      lookupHashTable ( const HashTable *table, StgWord key );
-void        insertHashTable ( HashTable *table, StgWord key, void *data );
 void *      removeHashTable ( HashTable *table, StgWord key, void *data );
 
 int keyCountHashTable (HashTable *table);